Initial Cell Search Method with MLD Based Frequency Offset Estimation in LTE Heterogeneous Networks

Abstract

This paper proposes a fast cell search method using maximum likelihood detection (MLD) based frequency offset estimation for a small cell in Long Term Evolution (LTE) heterogeneous networks (HetNets). In the proposed cell search method, a set of user equipment (UE) estimates the primary synchronization signal (PSS) timing and frequency offset using different PSS correlation computation methods in a macrocell or a small cell according to the employed carrier frequency. For a macrocell, we use the conventional cross- correlation based detection. Meanwhile, for a small cell with a high carrier frequency, a UE first detects the maximum autocorrelation peak between the received sample signals that are five milliseconds apart. Then, the UE estimates the fractional frequency offset from the phase rotation between two consecutive PSSs. After compensating for the fractional frequency offset, the UE simultaneously detects the accurate frequency offset that provides the phase rotation of 2PIxG (G denotes an integer) over the multiplexed interval of two PSSs and the PSS sequence based on MLD. After selecting the best cell, the UE detects the best secondary synchronization signal (SSS) sequence by taking the SSS sequence correlation for the received signal. System-level simulation results show that the proposed cell search method achieves a fast initial cell search time within approximately 120 ms to satisfy the physical cell ID detection probability of 95% even for the carrier frequency of a small cell up to 12 GHz, assuming the free- running frequency error of the UE standard oscillator of 3 ppm.